There is a great need to destroy a wide range of waste streams generated around the world and at the same time to convert this carbonaceous waste into useful hydrogen-rich syngas to accomplish one or more of the following: (1) production of renewable H2 fuel, (2) to feed an exothermic hydrocarbon synthesis reactor, such as a Fischer-Tropsch unit (FT), to produce renewable fuels, or (3) to use a portion of this syngas to drive a fuel cell to power the plant.
This steam/CO2-reforming chemistry used in the steam/CO2 reformer (SR) does not involve combustion. The idealized main chemical reactions, which can be considered to be chemical reduction, occur from the RR through the SSR main reactor is as follows:CaHbOc+d CO2+(a−c−d) H2O+HEAT→(a+d) CO+(a+0.5b−c−d)) H2 and many more reactions into the hundreds:C+2H2→CH4 CH4+CO2+HEAT→2CO+2H2 C+H2O→CO+H2 C+CO2-→2 CO+HEATH2+CO2<----- >H2O+CO. . . etc.
Although the steam-reforming chemistry is just started in the rotary reformer, it continues to near completion as the gas temperature increases along its path through the system into the main steam/CO2 reforming reactor where more reaction occurs with superheated steam preferably without catalysts. This reaction with superheated steam has a residence time of less than about two seconds mainly at the elevated, nearly isothermal conditions in main reactor; in some embodiments there is not the very brief exposure to high temperatures typical of an incinerator flame region.
The challenge and problem with fuel cells has been their extreme sensitivity to various unknown chemical poisons at parts per million levels coming from the waste streams from harming the electrochemical catalysts of the high temperature fuel cells. By comparison Flory-Huggins catalysts in Fischer-Tropsch reactors (such as supported iron and cobalt catalysts) are much less sensitive to poisons than fuel cells and are highly exothermic.CO+2H2→1/n(—CH2—)n(I)+H2O(I)ΔH°298=−231.1 kJ/mol
Conversion of syngas to methanol using copper catalysts in the gas phase or liquid-phase catalysts are exothermic and also less sensitive to poisons.CO+2H2→CH3OH(I)ΔH°298=−128.2 kJ/mol
There is syngas methanation that is highly exothermic:2CO+2H2→CH4+CO2ΔH°298=−247.3 kJ/mol
So these three reactions listed above cause the FT chemistry to produce many other gases besides the hydrocarbons [1/n (—CH2—)n (I)] desired. In some embodiments there is a minimization of these side reactions, or use of them, by reusing the FT tail gas through recycling back to the rotary reformer. The CO2 in Steam/CO2 reforming helps minimize this side reaction above.
All of these highly exothermic reactors produce high-grade useful energy, they all can convert syngas with enough exothermicity to make large amounts of electricity, steam and heat. These exothermic reactors can substitute for fuel cells. Thus, an aspect of some embodiments of the present invention include methods and process systems to convert waste to energy without burning the waste but to sequester the carbon of the waste so carbon gases are not released.
The composition of the syngas was determined in detail according to one embodiment of the present invention from a pilot plant where med-waste was steam/CO2 reformed to make syngas. The syngas composition is shown in Table 1 below.
TABLE 1Results from Pilot Plant Gas Test By Steam/CO2Reforming Of Solid WasteH2Hydrogen62.71vol %COCarbon Monoxide18.57CO2Carbon Dioxide10.67CH4Methane7.58C2H6Ethane0.48C3 TO C6Propane through hexane<0.01C6H6Benzene<17ppmCOSCarbonyl Sulfide4ppmCS2Carbon Disulfide0.05ppmH2SHydrogen Sulfide<5ppmC10H8Naphthalene2.6ppbC10H7CH32-Methylnaphthalene~0.6ppbC12H8Acenaphthalene~0.4ppbC12H8ODibenzofuran0.36ppbPCDF + PCDDPolychlorinated-0.0041ppt TEQdibenzofurans + Dioxins
What has been found experimentally was that the syngas included hydrogen and carbon monoxide. For fuel cells the key poisons, such as carbonyl sulfide, hydrogen sulfide, carbon disulfide, hydrogen chloride, and polychlorinated organics were identified. For Fischer-Tropsch, methanol synthesis, methanation, etc., this syngas is acceptable. However, to make the process more efficient and environmentally desirable, the side products, such as CH4 and CO2 need to be used and not released to the atmosphere. These are also the side products of FT that are called “tail gases.” Yet other aspects of some embodiments of the present invention include using these side species of CH4 and CO2 through recycle back to the front end of the process, such as through a rotary reformer.
Another aspect of power recovery is to reduce the energy losses of a waste-reforming kiln. One consideration is that some kilns are operated at a high temperature, followed by an even higher temperature steam/CO2 reformer which is then followed by the desulfurizer and high temperature filter—all energy-inefficient from heat losses from the process units themselves and from the complex of hot process piping.
Regarding Fischer-Tropsch, one aspect according to some embodiments is to develop a process train where the Fischer-Tropsch unit could produce enough high carbon product, such as fertilizer and as high density, unsaturated paraffin wax containing little hydrogen, so that the carbon in the waste feed would be sequestered in this product, without significant carbon emissions leaving the process anywhere else. The Fischer-Tropsch train also should produce steam for a steam-turbo-generator to make electricity for the process plant.